1. Field of the Invention
This invention relates to aqueous solutions or dispersions of hydroxyfunctional copolymers containing imide structural units and to a process for their production.
2. Description of the Prior Art
Binders based on acrylate copolymers dissolved in organic solvents in combination with crosslinking resins are distinguished by a number of excellent properties, including gloss retention and resistance to hydrolysis, weathering and yellowing. Other valuable paints are obtained by the incorporation of hydroxyl groups and are used in large quantities for paints, coatings, adhesives, printing inks and sealants.
To introduce hydroxyl groups, hydroxy-functional monomers, such as hydroxyethyl acrylates, are normally incorporated. To obtain high crosslink densities, relatively large quantities of hydroxy-functional monomers have to be incorporated such that the range of variation of the other monomers is restricted and, in addition, relatively expensive products are obtained.
Polyimides can be produced, for example, from bisanhydrides, such as pyromellitic anhydride, and from diamines. They are distinguished, inter alia, by excellent electrical insulation properties and by high long-term thermal stability. A serious disadvantage of the polyimides which are suitable for use on an industrial scale is their poor solubility. Large quantities of physiologically hazardous solvents, such as dimethyl acetamide, have to be used.
The economics of the coating compositions produced from the binders can be seriously impaired by the solvents used because the solvents are not incorporated in the coatings and have to be additionally eliminated, for example by burning, to reduce environmental pollution to the extent possible.
By contrast, water-based paints containing water dilutable or water soluble binders have the advantage that they contain relatively small quantities of organic solvents so that there is no need for expensive absorption units or furnaces.
An object of the present invention is to provide hydroxy-functional copolymers containing imide structural units dissolved or dispersed in water which are easy to prepare and, by virtue of a widely variable composition, would be suitable for various applications, for example in, or as, aqueous coating compositions, sealants or adhesives.
This object may be achieved by reacting special anhydride-functional and optionally carboxyl-functional copolymers with suitable aminoalcohols and optionally aminocarboxylic acids to form uncrosslinked hydroxy-functional copolymers containing imide structural units. These products can be diluted with water after at least partial neutralization of the carboxyl groups so that the copolymers are dissolved or dispersed in water.
That this object may be achieved is surprising because anhydride-functional copolymers are known to react with aminoalcohols to form crosslinked insoluble products. Crosslinking is caused by the anhydrides reacting very quickly with amino groups and also with hydroxyl groups to form amide or ester bonds.
This reaction principle is described, for example, in EP-A-48,128 for the production of two-component binders which cure rapidly to crosslinked products after mixing of the individual components.